docs: def find_links_to(links, node):
      """Find links to a node.
  
      :param links:
          forward links of a workflow
      :type links: Mapping[NodeId, Set[(NodeId, ArgumentType, [int|str]])]
  
      :param node:
          index to a node
      :type node: int
  
      :returns:
          dictionary of sources for each argument
      :rtype: Mapping[(ArgumentType, [int|str]), NodeId]
      """
      return {address: src
              for src, (tgt, address) in _all_valid(links)
              if tgt == node}